You totally understood my point. Just seems to me most people opt for either a medium duration tempo (40 minutes or under at LT pace) or a super long run like 90 minutes and over (without even training for half marathons).
Instead I keep it in the middle, and my double is acting as a legitimate long run with the recovery and adaptation (frequency) benefits, adding for 100 minutes.
It has been working for me so I'll keep doing it, no need to overcomplicate things.
Of course you can choose a different aproach.